We propose to design and develop a class of very potent and specific inhibitors of dihydrofolate reductase for use in the selective destruction of cancerous cells. Unlike the myriad inhibitors of this enzyme which have been prepared previously, this new class of compounds is intended to inhibit the enzyme on a functional, rather than structural basis, by acting as a transition state analog. The underlying principle in design of the analogs involves an adjustment of the reduction potential of the drugs so that they will bind extremely tightly and specifically to the target enzyme. Since the structures of the analogs will resemble that of the normal substrate closely, utilization of the analogs in clinical applications would not be dependent on the usual questions of drug pKa, transport, metabolism or elimination, relative to those of the normal substrate and therefore potentially represents an improved methodology for cancer chemotherapy. All potential chemotherapeutic agents will be screened for activity against dihydrofolate reductase from several sources, including normal and transformed human cells. Promising compounds will be tested in certain intact cells as well. In addition, these compounds will be made available to Dr. Harry Wood for testing in his cancer chemotherapy program.